1. Field of the Invention
The present invention relates to sputter coating systems and more particularly to sputter coating systems used for high volume production of relatively large size coated products.
2. The Prior Art
Extremely thin coatings can be applied to substrates by a process generally known as "sputtering." Sputtering is usually accomplished by bombarding a "target" formed from a desired coating material, such as gold, with ions so that individual atoms of the coating material are dislodged from the target and strike and adhere to the substrate. Generally the sputtering process is carried out under low pressure conditions (e.g., pressures of from 1 to 50 microns) with the target forming part of a cathode electrode from which electrons are emitted. An inert gas, such as Argon, is admitted to the vicinity of the cathode where the gas is ionized producing a plasma formed of positively charged gas ions and electrons. The ions are accelerated toward the target and strike the target with sufficient energy that target atoms are sputtered onto the substrate.
Some prior art sputtering systems employed so-called "post" electrodes in that the cathode was formed by an elongated post-like rod, or tube, or target material connected to a negative terminal of a D.C. power supply. Electrons emitted from these electrodes tended to disperse in all directions from the electrode and ionize the gas.
These systems were inefficient because in order to assure adequate ionization for sputtering the cathode had to be operated at high power levels and a relatively great number of gas atoms had to be present thus undesirably raising the pressure levels at which sputtering was accomplished. Secondly, sputtering tended to occur omnidirectionally from the electrodes and thus was not effectively directed toward the substrate being coated. Examples of systems employing such electrodes are disclosed by U.S. Pat. Nos. 3,738,928 and 3,414,503.
Efficiency of ionizing the gas was improved by the use of magnetic fields which were oriented to confine emitted electrons in regions close to the target surface. This technique, known as magnetic enhancement, was carried out by supporting magnets on the cathodes in orientations assuring that emitted electrons were constrained to remain relatively close to the surface of the sputtering target. Confining electrons adjacent the target surface causes increased numbers of collisions between the gas and the electrons thus increasing the ionization efficiency. This tended to reduce the cathode voltage requirements because fewer electrons had to be produced to accomplish a given amount of sputtering. Moreover a smaller quantity of gas could be admitted to the chamber to achieve the same amount of ionization. This permitted lower operating pressures with less interference between sputtered atoms and gas atoms in the enclosure.
In some proposals the shape of the magnetically enhanced cathode was such that the sputtering occurred primarily in a given direction. In one prior art construction the cathode employed a planar target plate having bar magnets supported beneath it in an annular array. The fields from these magnets arched over the target surface and collectively defined a "race track", or loop shaped region over the target surface in which the electrons tended to be confined. The magnetic fields also induced motion of the electrons in one direction through the region so that a continuous flow of electrons circulated around the track, or loop, in a relatively narrow band adjacent the surface of the target.
Gas molecules entering the electron flow were ionized thus forming a plasma in the region. The ions subsequently struck the target face adjacent the region. Atoms of the target material emitted as a result of the ion bombardment were uncharged and thus were substantially unaffected by the magnetic fields and the electrons during their flight to the substrate. This type of electrode construction is disclosed, for example, by U.S. Pat. Nos. 3,956,093 and 4,013,532.
In another proposed construction an annular target had magnets supported about its outer periphery so that emitted electrons were swept around the inner peripheral face of the target. A substrate placed within the target periphery was sputtered from all directions around the target periphery. See, for example, U.S. Pat. No. 3,878,085.
Cathodes of the character referred to tended to be eroded along narrow zones in the immediate vicinity of the electron flow path. This reduced the life of the target. A proposal was made in U.S. Pat. No. 3,956,093 for varying the elecron flow path in order to increase the size of the target erosion zone but substantial areas of the target still remained essentially uneroded.
Targets are frequently constructed from materials which themselves are extremely costly and/or are difficult and expensive to fabricate. Accordingly, localized severe target erosion is highly undesirable because it shortens the effective life of the target and requires relatively frequent electrode replacements. Moreover, in installations for quantity production of sizable sputtered products, such as architectural glass, replacement of electrodes in large enclosures necessitates venting the enclosures to atmosphere for replacement and subsequently pumping the enclosures back down to operating vacuum level. This is a time consuming and expensive interruption of production. Still further because the sputtering occurred over a small target area the sputtering rate was limited, thus limiting the production rate.
Using cathodes of the character referred to above was also somewhat ineffective for high volume production of articles such as architectural glass because only one face of one piece of glass could be coated at a time using one electrode. If, for example, two sheets of glass were to be coated simultaneously in order to increase production rates, two of the electrodes would have to be employed. A recent proposal has been made for constructing a two sided cathode formed by opposed separate target plates having a single array of magnets supported between them (e.g., as in U.S. Pat. No. 4,116,806). The magnets confine the electrons and ions to continuous loops extending over each target plate.